The provision of stonechip-resistant coatings on metallic substrates is of especial importance in the field of automotive manufacture. A surfacer or antistonechip primer is subject to a series of requirements. Hence the surfacer coat after curing is to bring about high stonechip resistance, more particularly in respect of multiple impact, and at the same time effective adhesion to the primer, more particularly to the cathodic electrodeposition coat (electrocoat for short) and to the basecoat, good filling properties (hiding the structure of the substrate) at coat thicknesses of about 20 to 35 μm, and good appearance in the context of the concluding clearcoat. Moreover, suitable coating materials, not least on environmental grounds, are to be preferably low in, or very substantially free from, organic solvents.
Coating materials for surfacers are known and are described in, for example, EP-A-0 788 523 and EP-A-1 192 200. Described therein are water-dilutable polyurethanes as binders for surfacers which are intended to ensure stonechip resistance, particularly at comparatively low coat thicknesses. On exposure in stonechip tests, however, in spite of good stonechip resistance, in other words a comparatively small number of instances of damage, the prior-art surfacers in OEM coat systems (electrocoat/surfacer/basecoat/clearcoat), nevertheless frequently exhibit damage patterns on the paint film where the unprotected metal substrate is exposed as a result of uncontrolled crack propagation in the OEM coat system and subsequent delamination at the interface between metal and electrocoat.
WO-A-01/04050 discloses inorganic anionic or cationic layered fillers for aqueous coating materials having good barrier properties, modified with organic compounds to widen the distance between the layers in the filler, said organic compounds having at least two ionic groups separated by at least four atoms. Cationic fillers employed may be mixed hydroxides, such as, more particularly, hydrotalcite types. The coating materials described in WO-A-01/04050 are used for coatings having very good barrier properties with respect to gases and liquids, the fillers being said not to affect the curing operation. The use of the coating materials to improve the damage patterns after impact exposure in OEM coat systems, more particularly for reducing the surface area of exposed substrate, is unknown. The coating compositions described in WO-A-01/04050 are of very limited suitability for use in OEM coat systems, since the multiple charge of the organic modifiers in the applied film produces a high local density of charges, which leads macroscopically to an increased hygroscopicity on the part of the cured coat, which has negative consequences in particular for the condensation resistance of the coat.
EP-A-0 282 619 describes solventborne anticorrosion coating materials comprising powderous mixed hydroxides, where anions used can be salicylate anions. The use of the coating materials to improve the damage patterns following impact exposure in OEM coat systems, more particularly for reducing the surface area of exposed substrate, is unknown.
M. L. Nobel et al. (Progress in Organic Coatings 58 (2007), 96-104) describe coating materials which can be used inter alia for OEM systems, comprising binders, crosslinkers, and aromatic fillers which have been modified with cationic organic compounds in order to widen the spacing of the layers in the filler. Cationic organic compounds of this kind are far less stable in aqueous phase than corresponding anionic compounds, and have a tendency, particularly in the case of the ammonium compounds to discolor when the coating material is cured, which can lead to unwanted shifts of shade in the coating. One feature emphasized is the accumulation of the modified inorganic fillers at the phase boundaries between droplets of dispersed polymer and water, or in the droplets, which is said to lead to an improved rheology and also to increased stiffness of the coats produced with the coating material. Generally speaking, an increase in stiffness in relatively thin coats leads to an increased tendency toward brittle fracture and hence to an increased exposure of substrate surface, and hence to an impaired damage pattern. The use of the coating materials described by M. L. Nobel et al. to improve the damage patterns following impact exposure in OEM coat systems, more particularly for reducing the surface area of exposed substrate, is not described.
In the light of the prior art, a problem which is left to be addressed by the present invention is the provision of stonechip-resistant coatings, based more particularly on environmentally advantageous aqueous coating materials, having a distinctly improved damage pattern, more particularly featuring a distinct reduction in the delamination of the integrated OEM coat system at the interface between metal and electrocoat, and hence featuring a distinct reduction in exposed substrate surface area after impact exposure. In preferred embodiments of the invention the stonechip-resistant coatings ought to exhibit a low tendency to absorb water and a low tendency toward discoloration when the coat is cured.